Microelectronic devices are often manufactured in and on silicon wafers and on other types other substrates. Such integrated circuits may include millions of transistors, such as metal oxide semiconductor (MOS) field effect transistors, as are well known in the art. MOS transistors typically comprise source, drain and gate regions, in which the gate material may typically comprise polysilicon. Polysilicon gates, however, can be susceptible to depletion effects, wherein an electric field applied to a polysilicon gate sweeps away carriers (holes in a p-type doped polysilicon, or electrons in an n-type doped polysilicon) so as to create a depletion of carriers in the area of the polysilicon gate near an underlying gate dielectric of the transistor. The depletion effect can add to the overall gate dielectric thickness in the MOS device. Recently, silicon germanium source and drain regions have been incorporated within transistors utilizing polysilicon gates, which greatly improves the performance of such transistors since the strained lattice of the silicon germanium regions enhance the electron and hole mobility within the channel of such a transistor, as is well known in the art.
Metal gates, on the other hand, are not as susceptible to depletion effects as gates comprising polysilicon. Typical prior art microelectronic processes, however, do not incorporate both metal gates and polysilicon gates within the same device or integrated circuit. This is due, in part, to the complexity and cost of developing a microelectronic process that can reliably form both a metal gate structure and a polysilicon gate structure within the same microelectronic device or integrated circuit. It would therefore be advantageous to incorporate both a metal gate structure and a polysilicon gate structure with silicon germanium source and drain regions. The methods and structures of the present invention provide such a process.